In the contemporary communication network, client-oriented access technology has always been a difficulty which restricts rapid development of high-bandwidth services. After arriving at the access network via transmission devices, the flood of information must be divided into streams to be interconnected with the terminal client. The bend-insensitive technology is a key link in the optical fiber access technology and a basic transmission medium of the optical fiber access network, can send information to the terminal client through complicated and changeable regions, and steadily realize high-bandwidth interconnection. It is classified by International Telecommunication Union as ITU-T G.657. G.657 bend-insensitive single mode optical fiber is mainly used in small and narrow space or at a corner, in a complex environment like a wire distribution box and an optical splitter, and in the field of optoelectronic devices, and still has a low additional bending loss with a bending radius equal to or smaller than 10 mm.
With the rapid development of optical fiber communication technology, optical fiber devices have been more and more widely applied in the optical communication field, among which optical fiber couplers have become the most-widely-applied passive optical fiber device. Optical fiber couplers play a vital role in realizing split combination, insertion, and distribution of optical signals. Optical fiber couplers are a multi-function and multi-purpose device and are one of the most important passive optical devices. In the developing process of optical fiber couplers, there are mainly three manufacturing methods: the etching method, the polishing and grinding method, and the fused biconical taper method. Among the three methods, the fused biconical taper method is most widely used thank to a simple operation, a low manufacturing cost, and a small device loss. Regarding all characteristic indexes, fused-biconical-taper optical fiber couplers are the most representative optical device.
As for traditional G.657 single mode optical fibers, the core thereof and the cladding thereof are designed with different materials, leading to a great difference in refractive indexes therebetween. Thus, in pursuit of a satisfactory macro-bending performance, a trench structure with a large proportion of fluorine and a deep sag of Δ is usually added in the design of an outer cladding. Therefore, during the fused-biconical-taper process, since material characteristics of the core and the cladding do not match (appearing as a mismatch in viscosity and in coefficient of thermal expansion), changes of an optical fiber waveguide structure do not synchronize, resulting in failure to reach a required optical splitting ratio no matter how technological conditions of fused biconical taper are optimized and a great loss during the process.
Although a common G.652 single mode optical fiber can satisfy the requirements for fused-biconical-taper performance, its bending loss is high when it comes to a small bending radius (smaller than 30 mm). Therefore, common G.652 single mode optical fibers are not suitable for being used in small-sized devices with special bending requirements.